Method of increasing the compressive strength of aluminum honeycomb core



Feb. 10 1970 J. H DEDRICK EVAL METHOD OF INCREASING THE COMPRESSIVE STRENGT494840 OF ALUMINUM HONEYCOMB CORE original Filed June so, 1964 n4! I A oojjooooooooio INVENToRs Zai 1% edrz'c Maffe/272319.99

mi@ MM ATTORNEYS United States Patent O 3,494,840 METHOD F INCREASING THE CGMPRESSIVE STRENGTH 0F ALUMNUM HONEYCOMB CORE John H. Dedrick and Musser F. Rupp, Henrico County,

Va., assgnors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Continuation of application Ser. No. 379,239, June 30, 1964. This application Dec. 14, 1967, Ser. No. 690,687 Int. Cl. C23b 9/02 U.S. Cl. 204-58 3 Claims ABSTRACT OF THE DISCLOSURE The rigidity and compressive strength of light gage aluminum articles initially having a thickness up to about 0.02 inch, such as thin walled honeycomb core, is increased by hard coat anodizing the aluminum to produce a coating thereon having a thickness of about to about 40% of the initial thickness of the aluminum prior to anodizing.

This application is a continuation of application Ser. No. 379,239, liled lune 30, 1964.

This invention relates to a novel method for increasing the strength of light gage aluminum, and to the resulting treated products. More particularly, the invention concerns a method for increasing buckling and dent resistance of aluminum cans and aluminum foil honeycomb by providing on the columnar surfaces of the aluminum a hard anodic coating, and to the resulting articles.

Thin walled structures of aluminum and aluminum base alloys have found increased applicability for food and beverage containers, building panels, aircraft wing construction, and similar uses. In these applications, the aluminum Walled articles are subjected to compressive stresses, which may result from axial loading, and in some instances, from laterally applied compressive loads.

Thus, for example, aluminum beer cans, made by extrusion or other means, are subject to large vertical loads, resulting from stacking cans on top of each other,

as well as to denting of can walls. Aluminum cans of this type conventionally have a wall thickness of about 10 mils (0.010 inch), and are about 4 to 5 inches high and about 2 to 2.5 inches in diameter.

Aluminum foil honeycomb is employed in airplane wing construction, and in making lightweight building panels in which it functions as a corestock between panels of other materials such as metal, plastic, glass or wood. The aluminum foil for this purpose may have a thickness of the order of 2 mils (0.002 inch). The honeycomb core comprises a plurality of corrugated strips of aluminum foil, parallel extending strips being bonded together node-to-node by a suitable adhesive, such as a thermosetting synthetic resin. A honeycomb core of this type is disclosed, for example, in U.S. Patent 2,828,235.

It will be apparent that both the can structures and the honeycomb cores are essentially columnar in structure, being composed of thin walled metal members which are long in proportion to their cross-sectional dimensions, and which are substantially straight and homogeneous in quality, and in which the compressive loads are axially applied. Under these conditions, the columnar structure, in analogy to a long pillar, will buckle and collapse under a load much smaller than that which would produce failure by crushing in a short piece of the same cross-sec- 3,494,840 Patented Feb. 10, 1970 ICC tion. Under axial loading, a critical load is reached which produces elastic instability, and until this critical load is reached, the member remains straight. This relationship may be expressed by Eulers Formula:

ing on end conditions, e.g. mi=4ll2 where both ends are rigidly fixed in position, Ezmodulus of elasticity, and

l/rzslenderness ratio. In general, the unit critical load (P/A) should be well below one-third of the ultimate compressive strength of the material.

In accordance with the present invention, it has been found that the compressive strength of thin walled aluminum structural members, such as food and beverage cans, and honeycomb cores, can be markedly increased by providing upon the surface of the aluminum a hard anodic coating. ln the case of cans, this treatment also results in greatly improved resistance to denting.

The term hard anodizing is well understood by those skilled in the art, and is explained in the standard reference Surface Treatment and Finishing of Aluminum and lts Alloys, by Wernick and Pinner (R. Draper, Ltd., 1956), Chapter 9, pages 311-312. In accordance with one standard employed in the anodizing art, the term hard coat is used to denote an oxide coating which, on 1100 aluminum alloy (commercial purity aluminum) will have a Knoop hardness number, taken in the cross section of the coating, of 450 KHN or more.

The hard anodizing of aluminum and aluminum base alloys is conventionally performed in aqueous electrolytes containing mineral acids or organic acids, or mixtures of such acids, generally at low temperatures in the range of 0 to 32 F. While conventional hard anodizing methods may be employed in accordance with the present invention, it has been found that superior results in terms of increased compressive strength, hardness of the anodic coating, and resistance to crazing, abrasion, and corrosion, are obtained by employing the novel hard anodizing process disclosed in copending application Ser. No. 353,591, filed Mar. 20, 1964, and accordingly, that process is preferred in the practice of the present invention. The preferred hard coat anodizing process comprises immersing the aluminum as anode in an aqueous anodizing electrolyte comprising (a) a mineral acid, such as sulfuric acid, (b) an organic acid, which may be either an aliphatic alpha-hydroxy monocarboxylic acid, such as hydroxyacetic acid, or an aliphatic dicarboxylic acid, such as oxalic acid, and (c) a metal salt of one of the aforementioned organic acids, and passing a direct current through the electrolyte at a current density between about l2 and about 60 amperes per square foot, at a temperature between about 50 F. and about 80 F. The method is applicable for the production of integrally colored anodic coatings, ranging from gold to dark brown. The sulfuric acid concentration is between about 0.05% and about 4.5% by Weight. The impressed voltage is gradually increased from about 20 volts to as high as 250 volts, depending upon desired tilm thickness, but generally up to about 60 volts.

The organic acids are employed in the anodizing bath in concentrations ranging from about 0.5% by Weight up to the limit of solubility. In addition to the acids named previously, other acids which may be used include lactic, malic, malonic succinic, and maleic acids. The metal salt component of the bath is a salt of a metal of Groups I-B and VIII of the Periodic System, for example, iron, nickel, cobalt, or manganese. The concentration of metal salt ranges from about 0.1% by weight up to the limit of solubility. n

The preferred anodizing electrolyte for the practice of the invention is an aqueous solution of sulfuric acid, oxallc acid, and ferrie oxalate, having the following range of cornposition, by weight:

Percent Sulfuric acid 0.05-4.5

Oxalic acid 0.50-9.0

Ferric oxalate 0.50-8.0

The average current density may be about 48 amperes per square foot, electrolyte temperature about 68-72 F., voltage in the range of about 35 to 60 volts D.C.

The aluminum may have a thickness up to about .020 inch; however, the thinner the gage of the aluminum member which is anodized, the more significant the anodic film becomes. Thus, in the case of `beer cans, having a thickness of about 0.0110 to 0.0135 inch, a hard coat of about 2.5 mils thickness (or about 20% of the original metal thickness) increases resistance to concentrated loads from 3 to 5 times that of the bare metal. In the case of aluminum foil honeycomb, where the foil thickness is, for example, about 2 mils (0.002 inch), the anodic coating is advantageously in the range of about to about 40% of the original thickness of the aluminum metal. The anodic coating may be applied to one or both sides of the metal. In the anodizing process a certain portion of the original metal thickness is consumed in the formation of the aluminum oxide film.

In accordance with another aspect of the invention, there are provided a novel arrangement of the anodizing apparatus and method of operation whereby the anodizing step can be performed on hollow columnar structures at greatly increased speed, and with enhanced uniformity of the anodic coating. In anodizing the interior of hollow objects such as cans and honeycomb cores fby conventional methods, the results are unsatisfactory for the reason that the thickness of the coating at the entrance of the openings tends to be greater than in the interior. The nonuniformity is attributable to the limited throwing power of the conventional anodizing bath arrangement.

In accordance with the invention, the problem of uniformity of coating has been solved -by employing as the cathode, one or more perforated metal electrodes. These electrodes may be fabricated in the form of a grid, or of a plate or sheet containing spaced openings permitting the circulation therethrough of the anodizing bath. Suitable metals are stainless steel, titanium, or aluminum. In conjunction with the use of the perforated cathode, agitation of the bath is employed to force electrolyte through or into the interior of the objects, such as cans or honeycomb cores, being anodized. Since the electrodes also are penetrable by the anodizing bath, thorough circulation of the bath and uniformity of deposition of the anodized coating are obtained. This enables the bath to be operated at elevated temperatures, even as high as 120 F., if desired, thus greatly increasing the speed of the process. At the same time it permits the optimum ratio of aluminum oxide film thickness to metal thickness to be maintained, and to prevent deposition of an oxide of such thickness that it would lbe defective and crack. The achievement of optimum film thickness produces the novel rigidizing effect of the invention, and the resistance to compressive deformation iwhich is an object of the invention.

In the accompanying drawings, the practice of the invention is illustrated in regard to the treatment of finished can bodies and honeycomb cores,

lt win be understood, however, that the aluminum can metal or the honeycomb strip or foil can be preanodized, in accordance with the invention, and thereafter fabricated into can bodies or honeycomb core stock.

In the drawings:

FIGURE 1 is a view in cross-section of an anodizing apparatus suitable for treating hollow objects such as cans or honeycomb cores;

FIGURE 2 is a plan view of the apparatus of FIGURE 1, `with one of the electrodes cut away to show the objects being anodized.

Referring to the drawings, the anodizing apparatus comprises a tank 10, rwhich may be made of suitable acidresistant material, which is provided with air agitating means comprising pipe 11, having openings 12 through which air under pressure is introduced into the tank 10 from a source not shown. It will be understood that several air agitating pipes may be employed.

The objects to be anodized, such as seamless aluminum cans 13, or honeycomb core 14 are mounted upon a supporting member 15, which is conducting, for example, a metal, and which is connected to a source of direct current as the anode. A pair of electrodes 16, which are interconnected by means of supporting bars 17, serves as the cathode, and is also connected to a D.C. source. The electrodes 16 are provided with openings 18, permitting free circulation of anodizing bath and agitating air therethrough.

The operation of the apparatus will be more fully understood by reference to the examples below, in `which the baths and the objects being treated are described, together with details of treatment.

The following examples illustrate the practice of the invention, but are not to be regarded as limiting.

EXAMPLE 1 Treatment of aluminum cans Three series of extruded aluminum beer cans were prepared, the can bodies being made of alloy No. 1070 consisting essentially of at least 99.70% aluminum. Thickness measurements were made at eight equally spaced locations around the body about one inch from the top. The can dimensions were approximately 2%6" diameter, and 4%" depth. The metal ranged in thickness from about 0.0114 inch to 0.0117 inch. The cans were degreased in alkaline detergent, rinsed in tap water, etched with 5% caustic soda 5 minutes at F., rinsed for 2 minutes, and then anodized in the apparatus shown in the drawing for 15 minutes in a bath having the composition:

rIuhe anodizing temperature was 70 F., the current denslty was 48 amperes per square foot. The voltage ranged from 33 initial to 60 volts direct current. The bath was agitated by compressed air. Coatings were obtained on three groups of cans, the first having a thickness averaging 1.0 to 1.1 mils, the second group about 1.6 mils, and the third group 2.4 to 2.8 mils.

The dent resistance of unanodized and anodized cans rwas then tested by applying a compressive load at the mid-point of the can body by means of an aluminum spherical nose, 1 inch diameter plunger. Four groups of c ans were tested, three series of 3 cans each representing light, medium, and heavy anodic coatings as described previously, and 3 cans unanodized, as extruded. Each group of cans was tested at loads o f 15 to 35 lbs. and then. unloaded, the load-unload curve being obtained aua tographically at 25 magnification. The loading portion of the curve (elastic plus plastic deformation) less 'the unloading portion of the curve (elastic recovery) was taken as the dent depth (permanent deformation) for each load. In order to obtain more points on the load vs. dent depth curve, several cans were rotated 180 to permit a second test on the opposite side.

The dent test results are summarized in Table l:

It will `be seen from Table 1, that at each loading value, the dent depth decreased progressively with increased thickness of the anodic coating, as compared with unanodized metal, ranging from a decrease of more than 80% at the 15-lb. loading to a decrease of 47% at the 35-lb. loading. These gures demonstate the remarkable increase in compressive strength imparted to the can bodies by the anodizing treatment of the invention.

EXAMPLE 2 Conventional hard coat anodizing applied to aluminum cans Certain specimens of 12 oZ. impact extruded beverage cans were chemically etched prior to anodzing, in order that the wall thickness after anodizing would correspond to that of the remaining untreated specimens. A hard anodic coating `was applied by conventional processing in an aqueous sulfuric acid electrolyte at 40 F. for 30 minutes at 25 amperes per square foot, producing a caating of about one mil (.001 inch) thickness on each of the opposite can surfaces.

Typical results were as follows:

TABLE 2 Dent depth, Thickness, inch inches Load, lbs. (permanent) Bare 0081 12. 0. 194 Anodized 0080 12. 5 0. 063

EXAMPLE 3 Treatment of honeycomb core Employing the apparatus depicted in the drawings, and the anodizing solution described in Example l, aluminum foil honeycomb was anodized, the bath being agitated by compressed air. The honeycomb metal was aluminum foil, having a thickness of 0.0018", the node face portions being fastened together with an epoxy resin. The honeycomb metal was Alloy No. 5056, having the following percentage composition limits: Si 0.30 max., Fe 0.40

max., Cu 010 max., Mn ODS-0.20, Mg 4.5-5.6, Cr' 0.05- 0.20, Zn 0.10 max., balance essentially aluminum. The honeycomb cells were approximately Pyle" in diameter, the core depth was l inch. All exposed cell surfaces were anodized with a coating having a thickness of 0.35 mil, the coating being uniform from top to bottom. The compressive strength of the honeycomb prior to anodizing averaged 255 lbs. to the buckling point. After anodizing the compressive strength was found to have risen to 383 lbs., an increase of almost 50%.

The compressive strength was tested by a modified ASTM method, involving placing a symmetrical 7-ce1l specimen between two metal plates and applying a compressive load, in standard testing apparatus.

A similarly treated 5056 alloy honeycomb, in depth, foil thickness 0.0018", cell diameter 3746", having an anodic coating 0.3 mil thick, showed an increase in compressive strength from 235 lbs. to 416 lbs. after anodizing, or about 75%.

The effect of anodic coating thickness on compressive strength of 46 honeycomb structures of various alloys and core depths is shown in Table 3:

TABLE 3.-EFFECT OF ANODIC COAT ON HONEYCOMB STRENGTH Percent increase compressive strength Depth Coat Core thickness mil.

Alloy No. (inch) 5056 (original metal 1.8 mils) 5056 (original metal 1.9 mils) 1 The remarkable strengthening of resistance to compressive forces as the anodic lm thickness i's increased is apparent from the data in the table.

While present preferred embodiments of the invention have been illustrated and described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

We claim:

1. Method of increasing the rigidity and compressive strength of a thin walled aluminum hnoeycomb core initially having a wall thickness on the order of 0.002 inch which comprises hard coat anodizing an aluminum surface of said core to produce an oxide coating thereon, said coating having a thickness of about 10% to about 40% of the initial thickness of the aluminum wall prior to anodizing.

2. Method of increasing the rigidity and compressive strength of a thin walled aluminum honeycomb structure initially having a wall thickness of about 0.002 inch which comprises immersing said structure in a hard coat anodizing electrolyte and hard coat anodizing the aluminum to produce an oxide coating of substantially uniform thick# ness on the surface thereof while agitating said electrolyte to force the same through the interior portions of said structure, said coating having a thickness of about 10% to about 40% of the initial thickness of the aluminum wall prior to anodizing.

3. A hard coat anodized aluminum foil honeycomb core produced by the method of claim 2.

(References on following page) References Cited UNITED STATES PATENTS Korpiun 204-58 Grouse et al 204-58 Waring 220-64X 5 Jones 161-68 Robinson 161-68 X Wagner 204--284 X Brenner et al 220-64 X 10 Feiner et al 204-284 X 8 3,265,239 8/1966 Kohan et al. 220-64 1,965,682 7/1934 Work 204-58 2,859,157 11/1958 Curtiss 204-26 OTHER REFERENCES Edwards, Junius D.: Anodic Coating of Aluminum, pamphlet received in Scientic Library, September 1939, pp. 8-9 and 13-15.

JOHN H. MACK, Primary Examiner RICHARD L. ANDREWS, Assistant Examiner 

